The universe keeps a tally of every newborn star. Over 13.8 billion years, galaxies have ignited so many stars that their 'census' can reveal the nature of the cosmos itself, just as tree rings reveal past climates.

Astronomers have gathered data on the star formation rate from today back to when the universe was less than half a billion years old (redshift 15). This curve, how many stars ignited at each moment, turns out to be sensitive to three key numbers: the fraction of ordinary and dark matter, the expansion rate (the Hubble constant), and the dark energy parameter. Individually, these quantities are hard to pin down: tweak one, and you can compensate with another, like balancing weights of unknown mass. But when the 'stellar chronicle' is combined with data on baryon acoustic oscillations (sound waves from the early universe, frozen into the distribution of galaxies) and Type Ia supernovae, the uncertainties shrink dramatically.

Combined analysis with fresh data from the DESI project yields an expansion rate of 68.28 ± 0.18 km/s per megaparsec. This is more precise than many classical methods and does not depend on assumptions about the early universe. Moreover, the method works even in models where dark energy is not constant but evolves over time.

Thus, star birth becomes more than just an object of study: it becomes a tool: a new ruler for measuring the cosmos, carved from the very history of light.